This invention relates to an aluminum electrolytic capacitor containing an electrolyte of a mono salt of dodecanedioic acid and a tertiary amine or a diproylamine as solute, water, and a mixture of ethylene glycol and N-methyl-2-pyrrolidinone as solvent, the electrolyte permitting capacitor operation at 130.degree. C. and 200 VDC or above.
Electrolytes for aluminum electrolytic capacitors operating at voltages of 200 V or higher most commonly contain salts of boric acid or boric acid derivatives in ethylene glycol. The maximum operating temperature for such an electrolyte system is less than 100.degree. C. and normally 65.degree.-85.degree. C. The temperature limitation is due to the rapid reaction of glycol with boric acid and other borate species to form polymeric glycol-borates and water at about 100.degree. C. The minimum operating temperature in such a system is above -20.degree. C., since glycol freezes at -17.4.degree. C.
The effective operating range can be expanded in both directions by replacing the glycol solvent with N,N-dimethylformamide (hereinafter DMF) which has a boiling point of 153.degree. C. and a freezing point of -61.degree. C. However, DMF is a very aggressive solvent that attacks most materials of construction. While the most resistant material for sealing gaskets and O-rings is Butyl rubber, DMF will be transmitted through a Butyl rubber closure at a rate which increases with increasing temperature and which limits the life of the capacitor, since the capacitor will not function adequately when the electrolyte loses half its solvent. This continuous slow loss of DMF also introduces a new difficulty, particularly if the capacitor is operating in a confined space, as the flash point of DMF is 67.degree. C.
In contrast glycol, which has a boiling point of 197.2.degree. C. and a flash point of 116.degree. C., is a much safer material, is much easier to contain, and its rates of transmission through both Butyl rubber and EPR rubber are almost negligible.
In current power supply applications, it is desirable to have an electrolytic capacitor operating at 200 VDC but capable of having superimposed on this DC voltage sufficient AC ripple voltage to raise the internal temperature to 120.degree.-125.degree. C. at an ambient temperature of 85.degree. C. An electrolytic capacitor that could operate continuously at 200 VDC at an ambient temperature of 130.degree. C. would meet the above high-temperature requirements.
The low-temperature requirements are much less stringent; it is likely that more than 90% would be met by a capacitor that retained 50% of its capacity at -40.degree. C. and 70% of its capacity at -20.degree. C. The requirements might, with some solutes, be met with an electrolyte in which glycol was the only solvent (other than water) and would certainly be met in an electrolyte in which glycol was mixed with an appropriate cosolvent. Such solvent systems are described by Ross and Finkelstein in copending application Ser. No. 22,554, filed Mar. 2, 1979.
Thus, it is desirable to develop an electrolytic capacitor capable of operating continuously at a voltage of 200 VDC or higher at an ambient temperature of 130.degree. C. and providing modest low temperature properties. However, the solute cannot be a borate, since borates react with glycol. In fact, the solute must be one which does not react chemically with either glycol or any other cosolvent that is used.
In addition, the solute must have excellent stability at the operating temperature, 130.degree. C., and good stability at somewhat higher temperatures. Thus, 150.degree. C. was chosen for screening purposes, and resistivity increases must be less than 25% after 1000 hr at 150.degree. C.
The major cause of resistivity increase, particularly where the solute is an ammonium or substituted ammonium salt of a monobasic or dibasic carboxylic acid is amide formation which converts conducting salt to a non-conducting amide. Generally, this reaction manifests itself through an increase in the resistivity of the electrolyte. Amide formation is easiest with ammonium salts, and the reaction occurs more readily with salts of primary amines than with salts of secondary amines. The reaction can even occur with salts of tertiary amines, although it is more difficult, since amide formation now requires cleavage of a carbon-nitrogen bond.
Another possible degradative reaction is ketone formation. This reaction is likely for the formation of C.sub.5 to C.sub.7 ketones, i.e. with salts of adipic, pimelic and suberic acids, but is of little consequence for the higher dibasic acids.
Electrolytes in which the solutes are amine salts of dodecanedioic acid meet the requirements given above. The diammonium salt of dodecanedioic acid has been disclosed in Japanese Showa 52-85356, and a solution of this salt in glycol-water is satisfactory in capacitors at 85.degree. C. Dodecanedioic acid can form both mono- and di-salts with amines, but the monoamine salts are considered to be more suitable. The iso-electric point, i.e. the point of maximum chemical stability and minimum solubility for aluminum oxide is at pH 5.5. Therefore, a slightly acid solute (i.e. a mono salt) is less likely to attack the aluminum oxide dielectric than a slightly basic solute (i.e. a di-salt). This consideration is of dominent importance at temperatures as high as 150.degree. C.